The invention relates particularly but not exclusively to a telephone system which includes a Subscriber Line Interface Circuit (SLIC), and the ensuing description will make reference to this field of application for convenience of explanation. This applicant has been particularly prolific in this area, acquiring several patents, for instance U.S. Pat. No. 5,461,670, and U.S. Pat. No. 5,706,343, hereby incorporated by reference.
A telephone system usually comprises a user outlet (i.e., a subscriber's telephone set), a connection element (such as a T-Tip and R-Ring pair), and a telephone exchange. The telephone exchange incorporates suitable equipment to serve all the basic functions of a telephone system, best known in the art as BORSCHT, an acronym for “Battery, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, Test”.
The features of these basic functions are well known to the skilled persons in the art and their discussion has been abbreviated or eliminated for brevity. However, some basic information is provided to frame the context of the invention.
The exchange is switched whenever a telephone subscriber dials a phone number. In particular, the number is sent to the exchange in either the form of pulses or audio tones; the exchange recognizes the dialled number and sends off a so-called “ringer” signal to inform the called phone set, i.e., the phone set that corresponds to the dialed number, of the existence of the call. Simultaneously with that ringer signal, an acknowledge signal is activated to inform the calling subscriber that the call is being processed.
Telephone sets have been equipped with signalling mechanisms of the above types from the very beginning. Specifically, the so-called “biased ringer” is still in use today which was invented by Watson in 1878; this signalling mechanism is actuated by an AC voltage having a frequency of 20 to 50 Hz and amplitude of about 90 Vrms, this amplitude being made necessary by the inefficiencies of magnetic materials and transmission lines which are parts of the telephone system.
A less stringent need of a substantial amplitude for the control signal of the call addressing signal still exists in current apparatus.
In particular, conventional sets employ an AC ringer signal which is usually produced at the telephone exchange by a DC motor driving an AC generator. Alternatively, this signal may be provided by an inverter, e.g. a solid-state inverter, capable of producing an AC voltage from a DC voltage, with no moving parts being used. In either constructions, a nominal exchange voltage of −48 VDC is typical.
The AC signal can then be sent to the line in a burst form modulated at a predetermined rate, either through the contacts of a relay controlled from a switch or directly from a suitable line interface.
The specifications for the ringer signal are provided by the regulatory bodies of the individual countries. A common provision relates to the issuing of the signal in either a balanced or an unbalanced manner.
Illustrated in FIG. 1 by way of example is the sending of a ringer signal in a balanced manner, and in FIGS. 2A and 2B the sending of the same in an unbalanced manner. In either of the above examples, the signal is placed on the line by means of a special relay 1, as is done in currently available apparatus.
It should be noted that, in conventional systems, the ringer signal is sent to the line in an unbalanced manner and overlaps the battery or supply voltage signal (typically of −48 VDC). However, this is impracticable where the apparatus are electronic throughout, due to the high voltages involved being difficult to control unless the silicon area to be given up to the integration of the whole system were expanded to a very large size, which is impractical to produce.
A prior approach to suppressing the ring relay consists of adopting a bridge configuration for the SLIC interface construction, and using for the ringer signal the same drive elements that are used for the other modes of operation of the telephone system.
In particular, a bridge type of SLIC interface provides for the ringer signal to be sent to the line in the balanced manner, being overlapped by a DC voltage of about 22 Volts effective to detect the trigger or off-hook signal from the telephone subscriber set.
FIG. 3 is a schematic diagram of a bridge type of SLIC interface which complies with established standards for both public and private applications on issues of amplitude and distortion of the issued signal.
Where very stringent requirements apply to amplitude and/or distortion, as is the case with short loop networks, the ringer signal can be generated directly from the SLIC interface utilizing the so-called reverse-polarity function. In fact, by using a low-level (0 logic level) square wave control signal having the same frequency as the ring frequency (typically 20 to 50 Hz), the drive elements can be driven as appropriate to obtain on the line a reversal of polarity at ring frequency, as shown schematically in FIG. 4.
In this case, the transactions can be controlled by means of a suitable capacitor which brings the line waveform close to a trapezoidal form, to thereby control the peak factor FC, given asFC=Vp/Vrms  (1) 
In this case, the balanced ringer signal shown in FIG. 5 is obtained.
Where compliance with the standards covering access to an ISDN network is required, such as Italy Telecom Technical Manual No. 1499, relating to large scale ISDN, it is important that the ringer signal be sent to the telephone line in an unbalanced manner and at a mean value equal to the battery level.